Material Characterization of the Costal Chondral Junction

Current models of the rib cage contain a discreet junction where the costal cartilage meets the rib bone. However, observations from a previous study find that the junction is not discreet and that this transition region varies in length among different subjects. Further investigation of this junction using a microCT scan found that the bone microstructure around the costal chondral junction extended above and below the cartilage midsubstance through the costal cartilage. The properties of this transition region could affect the frequency and location of rib fractures and be used to improve the current finite element models of the rib cage. This study investigates how the material properties of human ribs vary across costal chondral junction using microCT analysis along with ramp-hold, displacement driven indentation tests performed on the cartilage cross section and outer surface. INTRODUCTION ib fractures are the most common skeletal thoracic injury sustained by restrained occupants in frontal crashes (Pattimore et al., 1992). And studies have found that older drivers are more prone to chest injuries in automotive crashes (Morris et al., 2002). As a result, there has been a recent focus on the study of the rib cage including whole thorax mechanical (Kent et al., 2004), structural characterization of the rib bone (Charpail et al., 2005), and material level characterization of the rib bone through coupon tensile tests (Kemper et al., 2005). However, little work has been done to characterize the costal cartilage, which connects the rib bone to the sternum. The material properties of the costal cartilage could have affects on the mechanical response of the rib cage. R The prevalence of calcification in the costal cartilage increases with increasing age (Rejtarova et al., 2004). In addition, the most severe levels of calcification are more prevalent in the oldest age groups (Teal, et al., 1989). With the increasing number of older drivers, the calcification of the costal chondral junction could alter the effectiveness of the current restraint systems for these occupants. These regions of calcification could alter the mechanical response of the thorax and could lead to a source of stress concentration. This could increase the likelihood of obtaining rib fractures during frontal loading and could also alter the location of these rib fractures.